Capsule Endoscopy Explained

Capsule endoscopy is a non-invasive imaging technique in which a patient swallows a pill-sized wireless camera capsule that travels through the digestive tract by natural peristalsis, capturing thousands of images of the small intestine as it passes. Unlike conventional upper GI endoscopy and colonoscopy — which visualise the oesophagus, stomach, and large bowel but cannot access the majority of the small intestine — capsule endoscopy provides direct visualisation of the entire small bowel (jejunum and ileum), a region of the GI tract that spans approximately six to seven metres and was historically difficult to image without surgery. For patients with unexplained gastrointestinal bleeding, suspected Crohn’s disease of the small bowel, coeliac disease monitoring, or investigation of small bowel tumours, capsule endoscopy has transformed the diagnostic pathway by making direct small bowel mucosal imaging accessible in an outpatient setting without sedation, needles, or invasive procedures.

The capsule endoscopy system — most commonly the PillCam SB (Given Imaging/Medtronic) in clinical practice, though other systems including the Olympus Endocapsule and the Jinshan OMOM capsule are in use globally — consists of the capsule itself, a sensor array worn by the patient on a belt or vest, a data recorder that stores the images captured during the transit, and dedicated review software used by the gastroenterologist or radiologist to analyse the images. The capsule is approximately 11 × 26 mm, contains a CMOS camera, an LED illumination system, a battery lasting approximately eight to twelve hours, and a radio-frequency transmitter. It captures two to six frames per second and transmits images to the recorder worn by the patient in real time. The capsule is disposable — patients simply pass it naturally in the stool without noticing it.

What Capsule Endoscopy Is Used For

Capsule endoscopy is used primarily when a clinical question specifically concerns the small bowel — the region of the GI tract that upper GI endoscopy and colonoscopy cannot adequately visualise. The three most important clinical indications are unexplained GI bleeding, suspected Crohn’s disease of the small bowel, and coeliac disease surveillance or investigation of non-responsive coeliac disease.

Obscure gastrointestinal bleeding: This is the most common indication for small bowel capsule endoscopy. “Obscure GI bleeding” refers to gastrointestinal bleeding that persists or recurs after a negative upper GI endoscopy and colonoscopy — investigations that together cover approximately ninety-five percent of GI bleeding sources, with the remaining five percent originating in the small bowel. Small bowel sources of bleeding include: angioectasias (vascular malformations, also called angiodysplasia — the most common cause of small bowel bleeding in adults over sixty); Dieulafoy lesions (large, aberrant arterioles erupting through the mucosa); small bowel tumours (gastrointestinal stromal tumours [GISTs], lymphomas, carcinoid tumours, adenocarcinomas); Meckel’s diverticulum (a congenital remnant of the omphalomesenteric duct found in approximately two percent of the population, containing ectopic gastric mucosa that can ulcerate and bleed); NSAIDs-associated small bowel ulceration and erosions; and Crohn’s disease ulceration. Capsule endoscopy has a diagnostic yield of fifty to seventy percent in obscure GI bleeding — significantly higher than alternative small bowel imaging modalities for mucosal lesions. The stool tests that may indicate GI bleeding are covered in the stool tests for digestive health article, and the abdominal imaging context is discussed in the abdominal ultrasound and CT scan articles.

Suspected Crohn’s disease: When conventional endoscopy (ileocolonoscopy) and cross-sectional imaging (CT or MRI enterography) have not confirmed small bowel Crohn’s disease despite clinical suspicion, capsule endoscopy provides direct mucosal assessment of the proximal and mid small bowel beyond the reach of the colonoscope. Capsule endoscopy detects small bowel Crohn’s lesions (aphthous ulcers, linear ulcers, cobblestoning, strictures) with high sensitivity — though in patients with known or suspected small bowel strictures, capsule endoscopy carries a risk of capsule retention (described below). A patency capsule or cross-sectional imaging to assess for structuring disease is performed before small bowel capsule endoscopy in patients at risk of obstruction.

Coeliac disease: Capsule endoscopy can visualise the classic endoscopic features of villous atrophy (scalloping of mucosal folds, reduction in fold height, mosaic pattern) across the entire small bowel and may identify additional complications of coeliac disease — refractory coeliac disease, ulcerative jejunitis, small bowel adenocarcinoma, and enteropathy-associated T-cell lymphoma (EATL). The serology and duodenal biopsy-based diagnosis of coeliac disease is covered in the coeliac disease testing article; capsule endoscopy plays a complementary role in monitoring and complication detection rather than primary diagnosis. Small bowel polyps and tumours: In patients with polyposis syndromes — Peutz-Jeghers syndrome, familial adenomatous polyposis (FAP), and MUTYH-associated polyposis — capsule endoscopy is used for surveillance of small bowel polyps, guiding when and where balloon-assisted enteroscopy or surgical intervention is required. Other indications include investigation of protein-losing enteropathy, suspected small bowel Behçet’s disease, and portal hypertensive enteropathy.

Preparing for Capsule Endoscopy

Preparation for capsule endoscopy combines fasting with bowel preparation, though the preparation requirements are less stringent than for colonoscopy because transit through the small bowel (rather than the colon) is the primary focus. Standard preparation protocols vary between centres, but typically include: fasting from midnight before the procedure; a low-residue or clear liquid diet for the twenty-four hours before swallowing the capsule; polyethylene glycol (PEG) or sodium phosphate-based bowel preparation to clear the small bowel of food residue and improve image quality; simethicone (an anti-foaming agent) to reduce the small bubbles in luminal fluid that can obscure mucosal detail. Some centres use pro-kinetic agents (metoclopramide, domperidone) to ensure complete small bowel transit within the battery life of the capsule, particularly in patients with known or suspected delayed gastric emptying.

Medications: iron supplements and medications known to slow small bowel motility should be discussed with the referring clinician. Patients with cardiac pacemakers or implantable cardiac devices were historically advised against capsule endoscopy due to potential radiofrequency interference, though this restriction has been relaxed for most modern devices following safety studies — device-specific review should be performed before proceeding. Patients with known swallowing difficulty should alert the endoscopy team as the capsule can be endoscopically delivered into the stomach or duodenum if oral swallowing is not reliable.

What Happens on the Day

On arrival, the sensor array (a set of sensors attached to the patient’s abdomen with adhesive pads, or incorporated into a recording belt) is attached and connected to the data recorder worn on the waist. The patient swallows the capsule with a small glass of water. No sedation is required. After swallowing, patients are typically asked to fast for a further two to four hours (to allow the capsule to clear the stomach and enter the small bowel without food interference), after which a light snack is permitted. Activity restriction during the procedure is minimal — patients can walk and carry out gentle daily activities but are advised to avoid strenuous exercise (which can displace the sensor array) and MRI scanning during the recording period. Patients return to the endoscopy unit after eight hours to return the recorder; some units allow the recorder to be returned by post.

The gastroenterologist or radiologist reviews the recording using dedicated software — this typically takes thirty to sixty minutes for an experienced reader. The software displays the images at variable speed (typically four to twenty frames per second), allows navigation through the recording, and in modern systems incorporates artificial intelligence (AI) assistance that flags potential lesions for review. AI-assisted reading of capsule endoscopy images has demonstrated improved adenoma detection rates and reduced reading time in clinical trials and is increasingly integrated into routine capsule endoscopy practice. The final report is usually available within one to two weeks of the procedure.

What Capsule Endoscopy Can and Cannot Show

Capsule endoscopy is highly sensitive for mucosal lesions — small ulcers, erosions, angioectasias, and polyps in the small bowel that cross-sectional imaging (CT or MRI) will miss. Its key limitation is that it cannot take biopsies, cannot perform therapeutic interventions, and provides only luminal mucosal images without information about the bowel wall thickness or extra-intestinal structures. When capsule endoscopy identifies a lesion requiring biopsy or treatment, balloon-assisted enteroscopy (single or double-balloon enteroscopy, or spiral enteroscopy) is used — a technique that allows deep intubation of the small bowel using an overtube system, enabling biopsy forceps and therapeutic instruments to reach the target lesion under direct vision.

The capsule endoscopy report characterises the location of findings by the percentage of small bowel transit time rather than by anatomical distance (since actual small bowel length varies between patients and cannot be reliably measured during transit). A lesion at twenty-five percent transit time is in the proximal small bowel (duodenojejunal junction area); at fifty percent, mid-jejunum; at seventy-five percent, mid-to-distal ileum. This transit time percentage is used by the enteroscopist to plan the depth of insertion required when balloon-assisted enteroscopy is subsequently performed — deciding whether an oral (antegrade) or anal (retrograde via ileocaecal valve) approach will more efficiently reach the lesion.

Capsule Retention: The Key Risk

The principal risk of capsule endoscopy is capsule retention — the capsule becoming lodged at a small bowel stricture (narrowing) and failing to pass. Capsule retention occurs in approximately one to two percent of unselected small bowel capsule endoscopy procedures. In patients with known or suspected small bowel strictures — Crohn’s disease with fibrotic stricturing, NSAID enteropathy, radiation enteritis, ischaemic strictures — the risk is significantly higher (up to thirteen percent in some series). A retained capsule typically causes no immediate symptoms but can ultimately cause small bowel obstruction requiring surgical or enteroscopic retrieval.

To reduce the risk of capsule retention in patients with potential small bowel strictures, a patency capsule is used first: this is a dissolvable dummy capsule of the same dimensions as the endoscopy capsule that dissolves within thirty to sixty hours if retained in the bowel, avoiding the complication of mechanical obstruction. Patients swallow the patency capsule; if it passes normally (confirmed by plain X-ray or handheld scanner at forty-eight hours), the real capsule endoscopy can proceed safely. If the patency capsule is retained, small bowel stricturing is confirmed — balloon-assisted enteroscopy or surgical investigation becomes the appropriate next step rather than capsule endoscopy. In patients without risk factors for stricturing disease, routine patency capsule testing before capsule endoscopy is not required.

Capsule Endoscopy vs Other Small Bowel Investigations

The choice of small bowel investigation depends on the clinical question, the likelihood of therapeutic intervention being required, the patient’s comorbidities, and local availability. CT enterography (CT with oral contrast and enterography technique) and MR enterography (preferred over CT for younger patients and those requiring repeated monitoring due to the absence of radiation) provide cross-sectional imaging of the bowel wall and mesentery — superior to capsule endoscopy for identifying transmural disease, strictures, fistulae, abscesses, and extra-intestinal complications of Crohn’s disease, but inferior to capsule endoscopy for mucosal lesions. For patients with suspected Crohn’s disease, MR enterography and capsule endoscopy are often performed as complementary investigations, with MR providing transmural and structural assessment while capsule endoscopy provides mucosal detail. The MRI article in this series covers the principles of MRI in gastrointestinal assessment in detail. Push enteroscopy — a longer version of a standard gastroscope advanced beyond the duodenojejunal junction — visualises only the proximal one to two metres of the small bowel but enables biopsy and therapy and may be sufficient for lesions localised to the proximal jejunum. Balloon-assisted enteroscopy can reach the majority of the small bowel via oral or anal approach and provides full therapeutic capability — biopsy, haemostasis, polypectomy, dilation — but is a more prolonged procedure requiring sedation and a longer recovery period than capsule endoscopy.

Frequently Asked Questions About Capsule Endoscopy

Will I feel the capsule as it passes through?
No — the capsule is very small and smooth, and the vast majority of patients do not feel it at any point during transit. The gastrointestinal tract has no tactile sensation for a smooth intraluminal object of this size. Patients occasionally notice mild bloating or wind during the recording period, but this is related to the bowel preparation rather than the capsule itself. Some patients experience mild nausea when swallowing the capsule, though this is uncommon. The capsule passes in the stool, typically within twenty-four to seventy-two hours — most patients do not notice it in the toilet. If the capsule has not been confirmed as passed within two weeks, an abdominal X-ray may be taken to confirm its passage, though in practice this is rarely required in patients without symptoms of obstruction.

I’ve been told I need a capsule endoscopy for recurrent anaemia — what findings are they looking for?
Iron-deficiency anaemia that recurs after upper GI endoscopy and colonoscopy have excluded the most common sources (gastric ulcer, coeliac disease, colonic cancer, haemorrhoids) points toward a small bowel source as the explanation. The most common small bowel findings in this context are angioectasias — small, spider-like vascular malformations in the mucosa that bleed intermittently and are below the threshold of detection for CT or MRI in most cases. Capsule endoscopy detects angioectasias with high sensitivity and characterises their distribution and number, informing decisions about treatment with balloon-assisted enteroscopy (argon plasma coagulation of angioectasias) or medical management. Other findings in this context include small bowel ulceration from NSAID use, Dieulafoy lesions, and, in younger patients, a Meckel’s diverticulum. The liver function tests and full blood count context for anaemia investigation — including the serum ferritin, B12, folate, and coeliac serology that accompany a typical investigation — are covered across the relevant articles in this series. For patients with a confirmed finding on capsule endoscopy that requires treatment, balloon-assisted enteroscopy provides the therapeutic access that capsule endoscopy cannot, completing the diagnostic-to-therapeutic pathway initiated by the capsule study.

Sources: BSG — Capsule Endoscopy Guidelines · ASGE — Capsule Endoscopy Technology Review · NIDDK — Capsule Endoscopy